Electrolytes and Probiotics and Apple Cider Vinegar

I'm still new at chicken raising and want to be sure I'm doing the right things...

First some background:
I have 2 "sets" of chickens. The first set (6 red-variety) I got from TSC on March 1st and estimated them at 4 days old. The second set (2 hatched on June 2nd by a co-worker's son) were adopted on June 21st.

The 6 Red pullets (aka, Reds) are 23 weeks old and are all laying eggs daily now - one has been laying for 5 weeks already. They get Layer crumbles and forage daily, and I give them various treats (watermelon, grapes, carrot tops, berries, etc) and egg shells a few times a week when I'm putting them into their hen house in the evenings. They have an automatic waterer that we just installed a couple weekends ago.

The 2 EE pullets (from what the more experienced folks on this forum have told me - THANK YOU!) are now 9 weeks old and are referred to as the Chicklets. They get Grower/Starter crumbles and forage daily, and they too get various treats a few times a week. I still give the Chicklets electrolytes and probiotics in their waterer, but I'm wondering for how much longer...

I have the 2 sets separated until the Chicklets are larger, although I did some supervised integration a few times this past weekend.

So, here are my questions...
How long should I give chickens the electrolytes and probiotics? Should I still be mixing it into the automatic waterer for the Reds, or are they old enough to just have plain water? Should I supplement Apple Cider Vinegar into any of the water? Should the Reds get egg shells daily?

I wouldn't personally advise egg shells as this can cause them to eat the eggs they have laid. I give mine chicken grit that has oyster shell mixed in.
I have a flock of a mixture of ages and I put apple vinegar into their water once a week, daily if they catch a cold